1. Field of the Invention
The present invention relates in general to a hydraulic control valve with a regeneration function for heavy construction equipment such as an excavator, more specifically, to a hydraulic control valve capable of maintaining the pressure in a regeneration fluid passage, irrespective of changes in the discharge flow rate of a hydraulic pump, the location of working equipment, the regeneration flow rate and the return flow rate.
2. Description of the Related Art
Traditionally, a hydraulic control valve for use in heavy construction equipment, e.g., an excavator, had two functions: firstly, it controls the flow of pressurized fluid discharged from a hydraulic pump and supplies it to an actuator, e.g., a hydraulic cylinder; and secondly, it sends pressurized fluid returning during the operation of the hydraulic cylinder to a tank. With technical advances, a hydraulic control valve of recent years now has a regeneration function, whereby part of the pressurized fluid returning to the tank is supplied back to the hydraulic cylinder.
As aforementioned, the hydraulic control valve with a regeneration function sends the return pressurized fluid en route to the tank back to the hydraulic cylinder. In result, the work speed and the energy efficiency can be increased. Besides, it is possible to prevent the so-called cavitation phenomenon generated by insufficiency of a flow rate supplied to the chamber of hydraulic cylinder due to the transport speed of the hydraulic cylinder increased by the self-weight of the working equipment. Cavitation usually occurs at insufficiency of a flow rate into the chamber of the hydraulic cylinder and causes each component's life to be shorten. When actions for cavitation preventing are taken, the components' life can thus be extended.
FIG. 1 is a cross-sectional view of a related art hydraulic control valve with a regeneration function; FIG. 2 is a partial exploded view of a regeneration valve in FIG. 1; and FIG. 3 is a schematic view illustrating an example of the operation of a working equipment. The following will now describe the operation of a hydraulic control valve 3.
A pressurized fluid discharged from a hydraulic pump 1 is supplied to the hydraulic control valve 3 through a feed line (or supply line) 2. When a spool 7 inside the hydraulic control valve 3 switches to the left or right side, the pressurized fluid of the hydraulic pump 1 is supplied to a first port 4 or a second port 5 via a supply fluid passage 6.
The first port 4 is connected to a large chamber 8a of a hydraulic cylinder 8, and the second port 5 is connected to a small chamber 8b thereof. Hence, if the spool 7 switches to the right side, pressurized fluid is supplied to the large chamber 8a through the first port 4, and the hydraulic cylinder 8 is extended to the right side. Meanwhile, the pressurized fluid discharged from the small chamber 8b returns to a reservoir or a storage tank T by way of the second port 5 and a tank fluid passage 10b. 
If the spool 7 switches to the left side, pressurized fluid is supplied to the small chamber 8b through the second port 5, and the hydraulic cylinder 8 is retracted to the left side. Meanwhile, the pressurized fluid discharged from the large chamber 8a returns to the tank T by way of the first port 4 and the tank fluid passage 10a. 
When the spool 7 switches to the right side, as shown in FIG. 3, the hydraulic cylinder 8 is extended and an ‘arm-in’ is performed. Here, the pressurized fluid discharged from the small chamber 8b of the hydraulic cylinder 8 is supplied to the supply fluid passage 6 by the regeneration valve 12. Thus, by supplying part of this pressurized fluid en route to the tank back to the supply side (e.g., the hydraulic cylinder), it becomes possible to increase energy efficiency.
The pressurized fluid discharged from the small chamber 8b sequentially passes through the second port 5, first regeneration fluid passage 13, connection fluid passage 14, return fluid passage 16, and tank fluid passage 10b. Because the original area of the return fluid passage 16 is small, a pressure is created in the first regeneration fluid passage 13. If this pressure is higher than the pressure in a second generation fluid passage 15, a poppet 9 housed in the spool 7 moves to the right side, and the pressurized fluid of the first generation fluid passage 13 flows into the direction of the supply fluid passage 6 through the second regeneration fluid passage 15.
On the other hand, if a large force is required to operate the hydraulic cylinder 8, the pressure of the second port 5 should be lower than the pressure of the first port 4. That is, the lower the pressure of the first regeneration fluid passage 13, the larger the operational force for the hydraulic cylinder 8.
The regeneration spool 22 is elastically supported by a spring 23, and its front part comes in contact with a piston 21 which moves by the pressure from the supply fluid passage 6. Thus, if the pressure of the supply fluid passage 6 is higher than a designated value the piston 21 of the regeneration valve 12 is pressed by the pressure from the supply fluid passage 6, and the regeneration spool 22 moves to the right side. In result, the area of the return fluid passage 16 gradually increases and the pressure of the first regeneration fluid passage 13 is lowered accordingly, providing a larger force for the operation of the hydraulic cylinder 8.
The change of pressure in the first regeneration fluid passage 13, the flow rate passing through the first regeneration fluid passage 13 and the tank fluid passage 10b, and the area of the return fluid passage 16 satisfy the following equation:
      Δ    ⁢                  ⁢    P    =      C    ×                  (                  Q          A                )            2                      ΔP: The change of pressure in the first regeneration fluid passage 13;        C: Flow coefficient;        Q: Flow rate from the first regeneration fluid passage 13 to the tank fluid passage 10b; and        A: Variable area of the return fluid passage 16.        
Here, the flow rate Q is related to several variables, e.g., the feed rate of the hydraulic pump 1, the location of a working equipment shown in FIG. 3 for example, and the flow regenerated through the second regeneration fluid passage 15.
The change of pressure of the first regeneration fluid passage 13 and the pressure in the supply fluid passage 6 react sensitively to the flow rate Q and the area A. The change of the pressures in the first and second ports 4, 5 make the motion of the hydraulic cylinder unnatural and awkward (this phenomenon is called hunting) as a result of poor hydraulic stability. Therefore, it becomes very difficult to control the motion of the hydraulic cylinder 8.